1. Field of the Invention
The present invention relates to a pressure producing apparatus that produces a pressure applied to a liquid, e.g., an ink accommodated in an ink chamber of an ink jet printer.
2. Discussion of Related Art
There is known a piezoelectric-type pressure producing mechanism or device that is used to apply a pressure to an ink accommodated in a pressure chamber of an ink jet printer head. Such a device is shown in, e.g., FIG. 11 of Japanese Patent Document No. 2002-59547 or its corresponding U.S. Patent Publication No. 2002024567, or FIG. 6 of Japanese Patent Document No. 2002-127420 or FIG. 9 of its corresponding U.S. Pat. No. 6,530,880. FIG. 10 of the present application shows a cross-sectional view of a conventional ink jet printer head 101 including, as an actuator unit 106 thereof, a piezoelectric-type pressure producing device. In the printer head 101 shown in FIG. 10, the actuator unit 106 is driven by a drive pulse signal that is produced by a drive circuit, not shown. The drive pulse signal selectively takes a ground potential or a certain positive potential. The actuator unit 106 is stacked on a supply-passage unit 107 that defines a supply passage through which the ink is supplied. The actuator unit 106 and the supply-passage unit 107 are adhered to each other using an epoxy-type thermosetting adhesive. The drive pulse signal produced by the drive circuit is supplied to the actuator unit 106 from a flexible wiring board, not shown, that is bonded to an upper surface of the actuator unit 106.
The supply-passage unit 107 includes three metallic thin sheets, i.e., a cavity sheet 107a, a spacer sheet 107b, and a manifold sheet 107c, and additionally includes a nozzle sheet 107d that has nozzles 109 each for outputting ink and is formed of a synthetic resin such as polyimide. The four sheets 107a-107d are stacked on each other, such that the uppermost cavity sheet 107a is in contact with the actuator unit 106.
The cavity sheet 107a has two arrays of pressure chambers 110, arranged in a lengthwise direction thereof, each of which accommodates ink that is outputted when the actuator unit 106 is operated. The pressure chambers 110 are isolated from each other by partition walls 110a and arranged such that respective lengthwise directions of the pressure chambers 110 are parallel to each other. The spacer sheet 107b has a first communication hole 111 that communicates one end of each pressure chamber 110 with the corresponding nozzle 109 and additionally has a second communication hole, not shown, that communicates the other end of the each pressure chamber 110 with a manifold passage, not shown.
The manifold sheet 107c has a third communication hole 113 that communicates the above-indicated one end of each pressure chamber 110 with the corresponding nozzle 109. The manifold sheet 107c additionally has the above-indicated manifold passage, not shown, that supplies ink to the each pressure chamber 110. The manifold passage extends, beneath each array of pressure chambers 110, in the direction of arrangement of those chambers 110. One end of the manifold passage is connected to an ink supplying source, not shown. Thus, the manifold passage, the second communication hole, the pressure chamber 110, the first communication hole 111, and the third communication hole 113 cooperate with each other to provide an ink supply passage that supplies ink to the nozzle 109.
The actuator unit 106 includes six piezoelectric ceramic sheets 106a, 106b, 106c, 106d, 106e, 106f that are formed of a ceramic material, i.e., lead zirconate titanate (PZT) and are stacked on each other. In only a limited portion of the actuator unit 106 that corresponds to each pressure chamber 110 of the supply-passage unit 107, a first common electrode 121 is interposed between two piezoelectric ceramic sheets 106b, 106c, and a second common electrode 123 is interposed between two piezoelectric ceramic sheets 106d, 106e. In addition, in the same limited portion of the actuator unit 106, a first individual electrode 122 is interposed between two piezoelectric ceramic sheets 106c, 106d, and a second individual electrode 124 is interposed between two piezoelectric ceramic sheets 106e, 106f. 
The common electrodes 121, 123 are always kept at a ground potential, while the individual electrodes 122, 124 are supplied with the drive pulse signal. Respective portions of the three piezoelectric ceramic sheets 106c, 106d, 106e that are sandwiched by the four, common and individual electrodes 121, 123, 122, 124 cooperate with each other to provide an active portion 125 that is polarized, in advance, in the direction of stacking of the sheets 106c-106e when an electric field is applied thereto by the electrodes 121-124. Therefore, when the respective potentials of the two individual electrodes 122, 124 are changed to a certain positive potential, an electric field is applied to the active portion 125 of the piezoelectric ceramic sheets 106c-106e, so that the active portion 125 extends in the direction of stacking of the sheets 106c-106e. On the other hand, this phenomenon does not occur to the two piezoelectric ceramic sheets 106a, 106b. As a result, the active portion 125 of the actuator unit 106 swells out toward the pressure chamber 110. Thus, the volume of the pressure chamber 110 is decreased and accordingly a pressure is applied to the ink accommodated in the chamber 110, so that a droplet of ink is ejected from the nozzle 109.
FIG. 10 shows two pressure chambers 110. The left-hand pressure chamber 110 shows that a certain positive potential is applied to the two individual electrodes 122, 124, and the active portion 125 of the actuator unit 106 swells out toward the pressure chamber 110, so that the volume of the chamber 110 is decreased and accordingly a droplet of ink is ejected from the nozzle 109 communicating with the chamber 110. The right-hand pressure chamber 110 shows that the drive pulse signal is kept at the ground potential equal to the respective potentials of the common electrodes 121, 123, so that no ink is outputted from the nozzle 109 communicating with the chamber 110.
In addition, Japanese Patent Document No. 6-316070 shows, in FIGS. 1 and 2, an actuator unit of an ink jet printer head, i.e., a piezoelectric-type actuator unit having a so-called unimorph structure. This actuator unit includes a piezoelectric thin layer that is polarized in a direction of thickness thereof, and an electrically conductive coating layer and a flexible sheet that are bonded to the piezoelectric layer so as to sandwich the same. When an electric field is applied between the conductive coating layer and the flexible sheet, the piezoelectric layer swells out in the direction of thickness thereof and accordingly contracts in a direction parallel to a surface thereof. As a result, the piezoelectric layer swells out, with the flexible sheet, toward a pressure chamber. Subsequently, when the electric field is removed, the piezoelectric layer and the flexible sheet return to their initial, flat shape owing to their own elasticity. Thus, a droplet of ink is ejected from the pressure chamber.
All the actuator units disclosed by the above-identified three Japanese Patent Documents have a common feature that an electric field is applied to one or more piezoelectric elements in the direction of thickness thereof so as to deform the piezoelectric element or elements. Thus, it is required that the piezoelectric element or elements be sandwiched by the electrodes. However, complex steps are needed to manufacture an actuator unit having such structure, and accordingly the cost of manufacturing the same is increased.
In addition, the actuator unit disclosed by Japanese Patent Document No. 2002-59547 or Japanese Patent Document No. 2002-127420 has the structure that very thin piezoelectric sheets are stacked on each other. Therefore, if any of the piezoelectric sheets has fine cracks and accordingly ink leaks through the cracks, short circuit may occur between the two electrodes next to each other. This leads to lowering the durability of the actuator unit.